One offline question, how to understand this in function balance_pgdat:
/*
* Do some background aging of the anon list, to give
* pages a chance to be referenced before reclaiming.
*/
age_acitve_anon(zone, &sc);

The anon lrus use a two-handed clock algorithm. New anonymous pages
start off on the active anon list. Older anonymous pages get moved
to the inactive anon list.

If they get referenced before they reach the end of the inactive anon
list, they get moved back to the active list.

If we need to swap something out and find a non-referenced page at the
end of the inactive anon list, we will swap it out.

In order to make good pageout decisions, pages need to stay on the
inactive anon list for a longer time, so they have plenty of time to
get referenced, before the reclaim code looks at them.

To achieve that, we will move some active anon pages to the inactive
anon list even when we do not want to swap anything out - as long as
the inactive anon list is below its target size.